Most olefin polymers are currently manufactured using Ziegler-Natta catalysts, but single-site (metallocene and non-metallocene) catalysts represent the industry's future. Single-site catalysts can produce polymers with valuable physical properties such as narrow molecular weight distribution, reduced low molecular weight extractables, and modified melt rheology and relaxation characteristics. Traditional metallocenes incorporate cyclopentadienyl (Cp) ligands, as in bis(cyclopentadienyl)zirconium dichloride, but a variety of other “Cp-like” ligands have been used, including indenyl, fluorenyl, and substituted varieties of these.
Single-site catalysts that incorporate a transition metal and at least one “indenoindolyl” ligand are known. For example, U.S. Pat. No. 6,232,260 teaches the use of indenoindolyl Group 3-10 metal complexes as catalysts for polymerizing olefins. The examples illustrate the use of a non-bridged bis(indenoindolyl)zirconium complex for making high-density polyethylene. The '260 patent generally teaches that comonomers can be used in the polymerizations, that the complexes can be supported, and that the indenoindolyl ligand can be bridged to another ligand. The examples, however, are limited to unsupported, non-bridged complexes, so little is revealed about any advantages of using supported or bridged complexes. In fact, the non-bridged complexes have important limitations with respect to comonomer incorporation. We found that even with high levels of comonomer, it is difficult to push polymer densities below about 0.915 g/cm3 when a non-bridged indenoindolyl complex is used.
PCT Int. Appl. WO 99/24446 (Nifant'ev et al.) teaches metallocene complexes that incorporate a Group 3-6 transition metal and an indenoindolyl ligand. In many of the complexes, the indenoindolyl group is bridged to another ligand, which is often a second indenoindolyl ligand. The indene and indole rings are fused together in “[1,2-b]” or “[2,1-b]” orientation. (In the [1,2-b] ring system, the nitrogen of the indole ring is on the opposite side of the molecule from the methylene group of the indene ring; in the [2,1-b] ring system, the indole nitrogen and the indene methylene are on the same side of the molecule. Compare complexes 6 and 15 below).
Nifant'ev provides many examples of how to make bridged ligands and transition metal complexes that incorporate the ligands. The complexes are used to make high-density polyethylene (HDPE), linear low density polyethylene (LLDPE) having densities greater than 0.9 g/cm3, ethylene-propylene copolymers, and polypropylene. Nifant'ev uses only unsupported complexes, which have limited applicability for commercial processes such as the “slurry loop” process. Moreover, Nifant'ev teaches to use the unsupported catalysts with a high molar ratio of aluminum to transition metal, typically 1000-8000, for favorable activity. Unfortunately, the aluminum cocatalyst, because it is used in such a large excess, is often the most expensive catalyst component.
Still needed are commercially viable ways to make polyolefins using single-site catalysts. Preferably, the catalysts would incorporate an indenoindolyl ligand, which can be tailored with substituent variations to control catalyst activity and important polymer attributes such as melt index and molecular weight distribution. A preferred process would incorporate comonomers efficiently, thereby enabling the production of very low density polyolefins. Ideally, polyolefins with a wide range of densities from HDPE, to LLDPE, to very low density polyethylene and plastomers, could be made. Preferably, the process would provide access to ultra-high molecular weight LLDPE (i.e., Mw>200,000) having densities less than about 0.91. An ideal process would also allow polyolefin makers to control processability by regulating the amount of long-chain branching in the polymers. A valuable process would use a supported catalyst with commercial applicability to the slurry loop process, and would be active enough to use at low aluminum to transition metal ratios (i.e., less than 500 moles Al/mole transition metal).